There is a great deal of interest in techniques for photolyzing water into hydrogen and oxygen by using a semiconductor photocatalyst from the standpoint of light energy conversion, and the technologies have long been studied on basic research levels at universities and the like.
However, many studied examples involve many problems: for instance, (1) the activity is very low, (2) the reverse reaction is occurred largely, (3) the deactivation is caused quickly, (4) the preparation of the catalyst and the reaction conditions are troublesome, (5) the reproducibility is poor, and (6) it is doubtful whether the reaction proceeds catalytically. Therefore, these examples are far from the practical application.
Since the first (1973) and second (1978) oil crises the development of substitute energy for oil has been a pressing need among industrially developed countries.
Specifically, (1) atomic energy, (2) coal, natural gas, oil shale, and tar sand, (3) solar energy, water power, geothermal energy, wind force, and the like are studied. However, since recently the problem of the earth's environment is being highlighted, and in particular the major cause of warming of the earth is carbon dioxide given off from energy systems, the use of energy from the fossil sources mentioned under (1) and (2), not to mention oil, cannot help but decrease from now on. Therefore, attention is now being given to the so-called clean energy under (3).
Among these, solar energy alone is only considered enough to supply the amount of energy that is now used. For example, it is said that the solar energy reaching the earth in a year is about 2.8.times.10.sup.24 J, which is 10,000 times the amount of energy consumed by all people on the earth in a year.
To utilize solar energy practically, there are a method wherein solar energy is used as a heat source by a solar house or the like, and a method wherein solar energy is converted to other energy. The most advanced developed technique for solar energy conversion employs a solar cell, and the energy conversion efficiency of solar cells is on the order of 15%, but since the production cost of solar cells is high at present, energy problems cannot be solved only by solar cells.
It is said that the energy culture that will follow the current oil culture is a hydrogen culture. This is because if water is used as a hydrogen source, the resources are unlimited and give clean and high-power energy without giving off carbon dioxide. Hydrogen can be used in various ways, for example in fuel cells, in the synthesis of ammonia, in the purification of semiconductors, in the iron industry, in the food industry, as a fuel for automobiles and jet planes, and in the air-conditioning. To enable changing to a hydrogen culture, it is indispensable to establish an inexpensive method of the production of hydrogen in a large amount. Major techniques which are now under investigation include, for example, high-performance apparatuses for electrolyzing water, and thermochemical techniques that use a combination of several thermochemical reactions to decompose water while circulating reactants.
Among these studies for the production of hydrogen, there is a complete decomposition reaction of water using a photocatalyst, which can be directly combined with the use of the solar energy mentioned above. To overcome the energy problem using this reaction is a theme which should be pursued by human beings for ever, and it can solve the environmental problem at the same time.